1. Field of the Invention
The present invention relates to a serial recording apparatus and a recording method thereof for forming an image as moving a carriage mounted with a recording head.
2. Related Background Art
The print apparatus having the functions of printer, copier, facsimile machine, and so on, or the print apparatus used as an output device of composite electronic equipment including computers, word processors, and so on or workstation, is arranged to print an image on a printed medium such as paper or a plastic thin film, based on image information. Such print apparatus can be classified by their print method, for example, under the ink jet method, the wire dot method, the thermal method, the laser beam method, and so on.
In the print apparatus of the serial type adopting the serial scan method for primarily scanning the printed medium in directions intersecting the sheet carrying direction (the secondary scanning or sub-scan direction), the image is printed (or primarily scanned) by a print means mounted on the carriage moving along the printed medium, a predetermined amount of sheet feed (pitch carry) is carried out after completion of print of one line, thereafter the printed medium, again stopped, is subjected to printing (primary scanning) of a next line image, and this operation is repeated to effect recording on the entire printed medium. In the case of the print apparatus of a line type for recording the image only by secondarily scanning the printed medium in the carrying direction thereof, the printed medium is set at a predetermined print position, then a full line is printed together, then a predetermined amount of sheet feed (pitch feed) is carried out, a next line is printed together, and this operation is repeated to complete printing on the entire printed medium.
In order to eliminate band stripes in the width (of one line) of the print head, appearing upon scanning, the conventional print apparatus of the above serial type employs the fine print method in which the line feed pitch is set to the half to the quarter of the width of the print head, the dots forming the image are thinned out every scanning, and the dots are formed by a plurality of scanning steps of the carriage per line, thereby eliminating the band stripes.
In the above fine print method, however, forming positions of such adjacent dots are easy to deviate so as to become prominent in the image, because the adjacent dots are formed by plural scanning steps of carriage. It is thus necessary to secure the accuracy of dot forming positions in the plural scanning steps of carriage. It is, however, difficult to secure the accuracy of such dot forming positions, especially, when the carriage moves for cleaning of the recording head or the like, so as to change the start position of the carriage upon scanning. In addition to the problem upon the fine print, the problem of ruled line deviation or the like is likely to occur. To solve the problems, the following countermeasures have been taken.
(1) An encoder was mounted to detect absolute positions of the carriage thereby, thus securing the accuracy of accurate dot forming positions of image. This, however, was a cause of increase of cost.
(2) A stepping motor is often used to drive the carriage. In this case, the stepping motor is often used in the through region outside the self-starting region. Thus, it ramps up at low rotational frequency in the self-starting region and is accelerated up to a predetermined use rotational frequency. For stopping the motor, it is decelerated from the use rotational frequency to ramp down to a low rotational frequency in the self-starting region and to be stopped. The above drive method is usually used. Here, the distance for ramp-up was taken long enough to decrease a velocity change of the carriage at rotational frequencies during the print operation, thereby securing the accuracy of accurate dot forming positions of image. This, however, caused an increase of the apparatus size and an increase of the time necessary for printing.
(3) Further, the velocity change of the carriage was decreased by using the stepping motor of high resolution or adopting the microstep method as a driving method, thereby securing the accuracy of accurate dot forming positions of image. Such structure, however, was also a cause to increase the cost.
The reason why the structures as described in (1), (2), and (3) discussed above are taken is that there are possibilities that the scanning start position of the carriage deviates from that of the previous line because of the positional accuracy of the carriage and that color shear occurs in the case of black being made from three colors of yellow, cyan, and magenta.
An object of the present invention is to solve the above problems and thus to effect scanning by the carriage after the carriage is located at the start position, even with change of the carriage position before scanning.
Another object of the present invention is to set a difference of the start position of the carriage at every scanning to a distance equal to an integral multiple of a period of phase of motor.
The other objects of the present invention will become apparent in the description of specific embodiments to follow.